A construct consisting of a 1.5 kb segment of genomic 5' flanking sequence from the mouse Myc gene with the P1/P2 promoters, the first noncoding exon, a mouse Myc intron-less cDNA clone with an in-frame 6x histidine tag sequence on the 3' end, a short segment of the 3' UTR including the major polyadenylation signal, and a loxP site flanked neomycin resistance gene cassette was inserted in opposite transcriptional orientation into intron 1 of the C alpha locus via homologous recombination. The neo cassette was subsequently removed by mating to cre transgenic mice. (J:90901)